The present application relates to a circuit for assuring the turn-off of a power switching device and, more particularly, to a novel circuit for the self-commutated turning-off of latched power switching devices of the insulated-gate transistor/rectifier type.
It is now well known that the insulated-gate transistor, sometimes also referred to as an insulated-gate rectifier, has desirable power-control characteristics, in that (1) like a power field-effect transistor, it is a voltage-controlled device and (2) it has a saturation voltage drop similar to that of a bipolar power transistor, and lower than that of the typical power field-effect transistor. The insulated-gate transistor (IGT) currently has an undesirable latching characteristic, in that parasitic transistors in the IGT structure may latch into conduction and cause the loss of the ability to control the turn-off of the device main current flow by reduction of the device gate-source voltage. The device holding current, i.e. the collector-emitter current at which the IGT structure will unlatch with substantially zero gate drive, has been found to be at least two orders of magnitude below the level of current which causes the latching phenomenon to occur. Thus, a typical IGT device which might latch while conducting 20 amperes of collector current will not unlatch until the collector current falls below 0.2 amps, with a substantially zero gate-emitter voltage. This same IGT deVice, however, might unlatch at about 10 amperes of collector current if the gate drive is left turned on; however, retaining the gate drive in the turned-on condition is contrary to the desired turning off of the device. Thus, while use of one (or several parallelled) IGT, or IGR, to switch load power from a unipolarity source, of the types which might be characterized as a D.C. link-capacitor source, a high-ripple rectified-A.C. source and the like, or use of a plurality of such devices to switch load power from an A.C. bipolarity source, has distinct advantages over the use of the common power field-effect transistor with respect to power loss considerations, such advantages may be negated by the latching problem. Since latching may result from various phenomena (including, but not necessarily limited to, start-up current in-rush, supply transients, the too-rapid decrease of gate-emitter voltage and the like) when attempting to enter the turned-off condition, the power circuit designer often finds that the IGT-type device has latched and can therefore not be turned off by removing the gate electrode drive. In D.C. switching applications, the D.C. link current must collapse to less than about 1 percent of the latching current value, while in rectified or bipolarity A.C. circuits, the source waveform must nearly reach a natural current zero before the device will remove itself from the latching condition and turn itself off, after removal of the gate-emitter drive voltage. In the former case, a collapse to less than about 1 percent of the latching current value will almost never occur in a practical circuit. In any case, either the load or the IGT switching device may be destroyed by the excessive power dissipated during the excessive current conduction time introduced by latching of the IGT/IGR power switching device.
It is therefore highly desirable to provide a circuit capable of detecting when a power switching device of the IGT/IGR type is in a latched condition and for accomplishing a turning-off of that device at a much higher level of current than the normal holding current.